1. Field of the Invention
The present invention relates to processes for separation of alcohol and water to produce a dry alcohol stream.
2. Description of Background Art
The separation of methanol or other alcohols from water is accomplished in commercial applications primarily by distillation. While distillation separation is quite effective under the correct conditions, substantially dehydrating an alcohol from a wet methanol or other wet alcohol stream may be more complicated. When very dry methanol or other very dry alcohol is needed, the process may be inefficient. Specifically, a relatively large distillation column may be required with a larger number of distillation plates representing a high capital investment cost. There is a practical difficulty of drying methanol to a high degree of purity by distillation even for non-azeotropic forming alcohols or alcohols that don't form constant boiling mixtures with water that requires a significant heat requirement resulting in higher operation costs, too. By wet alcohol, it is meant alcohol that has no more than 7 wt. % water and at least 0.5 wt. % water, based upon the total weight of the alcohol stream. By dry alcohol, it is meant alcohol having no more than 0.5 wt. % water, based upon the total weight of the alcohol stream. Thus, in the region where a feedstock containing methanol and other alcohols has an alcohol concentration already greater than 92 wt. % and higher levels of purity are required, distillation may be prohibitively expensive. Thus, there is a need for alternative methods of drying a wet alcohol stream—including a wet methanol stream.
A process that could potentially benefit from a more efficient process for separating water from an alcohol stream is biodiesel production. Biodiesel is produced in two main reactions esterification and transesterification. Methanol and other alcohols can be used as reactants in the production of both esterification and transesterification reactions provided that they are free of water. To promote efficient conversion, these reactions require the use of substantial quantities of excess methanol and other alcohols, which have to be removed in other parts of the process.
Animal and plant fats and oils are typically made of triglycerides which are esters of free fatty acids with the trihydric alcohol, glycerol. The production of biodiesel involves a reaction called transesterification. In the transesterification process, the alcohol is deprotonated with a base to make it a stronger nucleophile. It reacts in the presence of methanol to form glycerol and three methyl esters of the fatty acids.
This reaction for transesterification is written as follows:
wherein, R1, R2, and R3 in this diagram represent long carbon chains.
The only reactants in this reaction are the triglycerides and the alcohol. The reaction is endothermic. Therefore, under normal conditions, this reaction will proceed exceedingly slowly or not at all. Application of heat and the use of a base catalyst help increase the rate of the reaction. This reaction is also a reversible reaction. Thus, higher concentrations of methanol (or other alcohol) in the reaction chamber is will result in a higher methyl ester product yield.
The excess, unreacted methanol or other alcohol required to push the reaction to greater product yield remains in the reaction mass (methyl esters and glycerol) that is produced. For economic and product quality reasons, this methanol or other alcohol must be recovered from the product stream for an economically viable use. In order to reuse this recovered methanol in a transesterification reaction, it must be dry methanol or other dry alcohol.
Excess methanol or other alcohol in the transesterification section partitions to both biodiesel (3-5 wt. % methanol to strip) and glycerin (about 25-30 wt. % methanol to strip). If water is present in this reaction, it causes the reaction to produce a larger percentage of glycerin and less biodiesel.
Furthermore, biodiesel feed stocks inevitably contain some quantity of free fatty acids. These compounds, when present in the transesterification reaction, form soaps from the FFAs by reacting with the basic catalyst and produce water as a bi-product. Free fatty acid levels in the transesterification reaction feedstock that are higher than 1% create sufficient soap and water to cause significant operational and quality problems in a biodiesel plant. Reacting the free fatty acids in a process called esterification prior to the transesterification reaction is one way to prevent soap formation in the transesterification reaction. With appropriate catalysts (such as sulfuric acid), methanol and free fatty acids react to form methyl esters of the free fatty acids. Substantial excess methanol is used in this reaction to drive the reaction towards completion and improve methyl ester product yield. Water is a byproduct of this reaction. The esterification reaction is represented by the following Reaction 2:

Since water hinders the transesterification reaction, it should be removed from the product mixture following esterification but before transesterification is performed. Separating the water from the fatty acid methyl esters, oils and triglycerides can be accomplished by evaporation. But to recover the excess methanol in a dry usable form is expensive. Whatever biodiesel process is employed, removal of water from the recycled methanol or other alcohol is essential if the excess methanol or other alcohol in the product stream is to be recovered in a suitable quality for reuse in the reaction vessel.
During the recovery process of the biodiesel, methanol or other alcohol and water is removed from the biodiesel product, as a first step, resulting in a methanol or other alcohol stream that contains a concentrated level of water—about 1.5 wt. %. Presently, the typical technique for alcohol removal is either simple distillation by single stage flashing or complex distillation with a multistage distillation column.
Flashing is a very simple process and can be effective, but it does not yield a pure alcohol stream if water is present. Recycle and reuse of the alcohol in the reaction vessel is not feasible for this separation method. Multistage distillation can separate alcohol and water to a sufficiently high level of purity for recycle and reuse in the reactor, but a very large and energy intensive distillation column is required to produce the dry alcohol. A distillation column in this application is operationally challenging and is sensitive to changes in feed composition. A system that would purify alcohol, be operationally simple and be rather indifferent to feedstock composition would be a very useful for biodiesel plants. The present invention addresses these and other concerns.